332 DIY Science: Illustrated Guide to Home Chemistry Experiments
SBSTITUTIU oNS ANd modIfICATIoNS- An inoculating loop is simply a length of inert wire
(usually platinum or Nichrome) with a tiny loop at the
end, held in a glass or metal handle. You can make
your own by forming a loop at the end of a chrome-
plated paper clip and using a wooden clothespin as a
handle. If you do not have an inoculating loop, you may
substitute strips of paper, toothpicks, or wood splints.
Soak the paper or wood in a concentrated solution of
each sample and allow it to dry. Hold the dried paper
or wood in the burner flame and observe any color
imparted to the flame. - You may substitute a dark blue photographic filter for
the cobalt glass square. - You may substitute hardware-store muriatic acid
of at least 30% concentration for the concentrated
hydrochloric acid. (But note that hardware store acid
may contain impurities that will affect the tests.) - Obtain samples of as many metal ion species as
possible, including barium, boron (use boric acid),
calcium, copper(I), copper(II), iron(II), iron(III),
lead, lithium, magnesium, manganese, potassium,
sodium, strontium, and zinc. You need only a few
grains of each sample as a solid or a small amount of
the sample in solution. Try to obtain the samples as
nitrates or chlorides, which vaporize more readily than
carbonates, hydroxides, oxides, sulfates, and salts with
other anions.
LABORATORY 19 .1:
USE fLAmE TESTS To dISCRImINATE mETAL IoNS
It has been known since antiquity that the
presence of certain metals causes a flame to
assume characteristic colors. For example,
the presence of sodium causes a flame to
assume a bright yellow color, and the presence
of copper tints the flame a striking blue-
green. This phenomenon occurs because heat
excites atoms, boosting the outer electrons
of their shells to higher energy states. As
those electrons return to their original, lower
energy states, the excess energy is released as
photons at specific, characteristic wavelengths.
The flame test uses this phenomenon to
determine whether specific elements are
present in a sample.
RIREEqU d EqUIpmENT ANd SUppLIES£ goggles, gloves, and protective clothing£ test tube£ test tube rack£ gas burner£ inoculating loop (platinum or Nichrome)£ cobalt glass square£ hydrochloric acid, concentrated (~10 mL)£ samples (see Substitutions and modifications)In theory, the flame test can discriminate numerous elements
by their flame colors, some of which are listed in Table 19-1. In
practice, it’s a bit more difficult. The first problem is ambiguity.
If a flame test shows a bright red color, for example, does the
sample contain lithium or strontium (or both)? Does a pale violet
color indicate the presence of potassium in the sample, or is it
cesium? Is the flame pale green (antimony or barium), bright
green (boron), yellow-green (manganese or molybdenum), or
just plain green (copper)? The second problem is masking. Some
elements, notably sodium, produce an intense flame coloration if
they are present in even tiny amounts. Most elements produce a
much subtler coloration, which may be overwhelmed by the more
intense color of another element.
So is flame testing useless for real qualitative analytical work? Yes
and no. In a home laboratory, flame tests are useful primarily to
illustrate the principles involved. There are much more accurate
methods available for qualitative analysis of metal ions in a
home lab, which we’ll explore in later sessions. Conversely, in
professional laboratories, flame testing is one of the primary
methods used for qualitative inorganic analysis. Professional labs
use flame spectrometers, expensive instruments that can detect
and unambiguously identify elements at the parts-per-million
level by charting the emitted spectrum of a sample.
We don’t have a flame spectrometer, and you probably don’t
either, so we’ll use the traditional flame test technique in this lab.